1. Field of the Invention
The present invention relates to a flat non-aqueous electrolyte secondary cell and in particular to a flat non-aqueous electrolyte secondary cell with improvements in heavy loading discharge characteristics.
2. Description of the Prior Art
In recent years, there are commercially available coin- or button-shaped flat non-aqueous electrolyte secondary cells wherein metal oxides such as MnO2 and V2O5, inorganic compounds such as fluorinated graphite, or organic compounds such as polyaniline and polyacene structural compounds are used as the cathode active material, while metal lithium or lithium alloys, organic compounds such as polyacene structural compounds, carbon materials capable of occluding and releasing lithium, or oxides such as lithium titanate or lithium-containing silicon oxides are used in the anode, and non-aqueous electrolytes containing a supporting electrolyte such as LiClO4, LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2 and LiN(C2F5SO2)2 dissolved in a non-aqueous solvent such as propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane and y-butyl lactone are used as the electrolyte. These cells are used as power sources for backing up SRAM and RTC where an electric current is discharged for light loading of about several to dozens ALA, or as main power sources for wristwatches not requiring cell exchange.
In general, these coin- or button-shaped flat non-aqueous electrolyte secondary cells have the structure shown in FIG. 4. That is, a metallic anode case 5 also serving as an anode terminal and a metallic cathode case 1 also serving as a cathode terminal are fit to each other via an insulating gasket 6, and further the cathode case 1 has a sealed opening structure caulked by caulking, and in the inside of this structure, tablet-shaped cathode 12 and anode 14 having a smaller diameter than the opening of the insulating gasket 6 are set up against each other via a single- or multi-ply separator 13 impregnated with a non-aqueous electrolyte.
The coin- or button-shaped flat non-aqueous electrolyte secondary cells as described above have the advantage that they are easily producible, excellent in mass-productivity, and superior in long-term reliability and safety. Further, by virtue of their simple structure, the most distinctive feature of these cells is that their miniaturization is feasible.
Meanwhile, the miniaturization of devices (mainly compact information terminals) such as portable telephones and PDA is promoted, thus making it essential to miniaturize secondary cells as their main power sources. In these power sources, there have been used cylindrical or rectangular alkali secondary cells such as lithium ion secondary cells wherein lithium-containing oxides such as lithium cobaltate is used as the cathode active material while a carbon material is used in the anode, or nickel hydride secondary cells wherein nickel oxyhydroxide is used as the cathode active material and a hydrogen-occluding alloy is used as the anode active material. These cells have been constructed by coating or filling a current-collecting body consisting of a metal foil or metal net with an active material layer to form an electrode, then welding a tab terminal into the center of the electrode, and winding or laminating it to form an electrode group, complicatedly bending the tab terminal from the center of the electrode group and welding the terminal into a safety element, an opening-sealed pin or a cell can. However, these cells have been constructed in such a complicated process that they are inferior in workability and the miniaturization of parts therein is also difficult. Further, these cells should be provided therein with a space for preventing the tab terminal from short-circuiting or for integrating a large number of parts such as safety element into the cells, and thus there is a limit to the miniaturization of these cells at present.
For miniaturization of the cells under these circumstances, the present inventors have attempted not at miniaturizing cylindrical or rectangular lithium ion secondary cells or nickel hydride secondary cells, but at achieving a higher output of the flat non-aqueous electrolyte secondary cells described above. That is, the present inventors have used lithium cobaltate of high capacity and high potential as the cathode active material and a graphitized carbon material of high capacity excellent in voltage evenness as the anode active material, and according to the process and structure of the conventional flat non-aqueous electrolyte secondary cell, the inventors have processed the cathode and anode into tablets smaller than a gasket, to prepare a cell.
However, this cell though attaining superior characteristics to the conventional flat non-aqueous electrolyte secondary cell is not satisfactory when discharged in a large current required of a main power source in compact portable devices, thus failing to achieve levels satisfactory as a main power source in compact portable devices. Accordingly, the development of techniques for permitting the heavy-loading discharge characteristics of the compact flat non-aqueous secondary cell to reach levels not achieved in the prior art is necessary.
This invention was made in view of the circumstances described above, and the object of this invention is to provide a flat non-aqueous electrolyte secondary cell, which is remarkably superior in heavy-loading discharge characteristics.
The present inventors made extensive study on the improvement of the heavy-loading discharge characteristics of the flat non-aqueous electrolyte secondary cell described above. As a result, they found that the heavy-loading discharge characteristics are significantly improved by allowing the area of the electrodes to be significantly larger than in the conventional flat non-aqueous electrolyte secondary cell, to arrive at the present invention.
That is, the present invention relates to a flat non-aqueous electrolyte secondary cell comprising a metallic anode case also serving as an anode terminal and a metallic cathode case also serving as a cathode terminal fit to each other via an insulating gasket, the anode or cathode case having an opening-sealed structure caulked by caulking and having in the inside thereof an electricity-generating element including at least a cathode, a separator and an anode and a non-aqueous electrolyte, wherein a plurality of electrode units each consisting of the cathode and the anode opposite to each another via the separator are laminated to form an electrode group, or a sheet-shaped electrode unit consisting of the cathode and the anode opposite to each another via the separator is wound to form an electrode group, or a sheet-shape cathode is wrapped with the separator except for a part contacting at inner face of cathode case and a sheet-shaped anode is set on the sheet-shaped cathode in a right angled position each other and then these cathode and anode are bent alternately to form an electrode group, and the total sum of the areas of the opposing cathode and anode in this electrode group is larger than the area of the opening of said insulating gasket.
Further, the present invention relates to a flat non-aqueous electrolyte secondary cell comprising a metallic cell case also serving as an electrode terminal, an opening-sealing plate for sealing an opening in said cell case, and another electrode terminal arranged via an insulator in an opening provided in a part of the opening-sealing plate, said cell case being provided inside with an electricity-generating element including at least a cathode, a separator and an anode and a non-aqueous electrolyte, wherein an electrode group consisting of an electrode unit having the cathode and the anode opposite to each another via the separator is formed, and the total sum of the areas of the opposing cathode and anode in this electrode group is larger than the area of the opening of said opening-sealing plate.
As the forms where the total sum of the areas of the opposing cathode and anode in the electrode group is larger than the area of the opening of the insulating gasket in the present invention as described above, there is the form (1) wherein a plurality of the above-described electrode units are laminated to form an electrode group, and the total sum of the areas of the opposing cathodes and anodes in this electrode group is larger than the area of the opening of the insulating gasket, or the form (2) wherein the above electrode units are in the form of a sheet, and said sheet-formed electrode units are wound to form an electrode group, and the total sum of the areas of the opposing cathodes and anodes in this electrode group is larger than the area of the opening of the insulating gasket, or the form (3) wherein a sheet-shape cathode is wrapped with the separator except for a part contacting at inner face of cathode case and a sheet-shaped anode is set on the sheet-shaped cathode in a right angled position each other and then these cathode and anode are bent alternately to form an electrode group, and the total sum of the areas of the opposing cathodes and anodes in this electrode group is larger than the area of the opening of the insulating gasket.
As described above, the total sum of the areas of the opposing cathodes and anodes in the electrode group is made larger than the area of the opening of the insulating gasket or the opening-sealed plate, whereby the heavy-loading discharge characteristics of the flat non-aqueous electrolyte secondary cell can be significantly improved.